The present invention relates to a bobbin case for accommodating a lower-thread bobbin by setting it in the rotary hook base of a rotary hook portion in a lock stitch sewing machine and a lower-thread bobbin.
As a bobbin case to be set in the rotary hook base of a rotary hook portion in a lock stitch sewing machine, there are known two types: a so-called DB type of FIG. 3 in which a bobbin case is rotated counterclockwise as viewed from a rotary hook portion and a needle is located in the rear of a hook base and in front of a beak of a rotary hook body, and a so-called DP type of FIG. 7 in which a bobbin case is rotated clockwise as viewed from a rotary hook portion and a needle is located in the rear the beak of rotary hook body and in front of a needle guide provided on the back surface of the rotary hook. Members having functions common to both types are given like reference characters.
In these bobbin cases, an open groove 21a obliquely extending up to an edge portion is formed in the peripheral wall portion 21b of a body 21. Further, a tension spring 22 which has a thread guide portion 22a at its free end and whose spring pressure is adjustable with a screw 23 is fixed to the outer periphery of the peripheral wall portion 21b of the body 21. Further, a latch 24 for retaining the body 21 to a stud uprightly provided in the center of a rotary hook base (not shown) is installed on the outer face of the bottom portion 21c of the body 21.
In any one of these types, a general-type lower-thread bobbin 25 shown in FIG. 7 is installed inside and bobbin thread Y is wound on such a bobbin by a lower-thread winding unit (not shown) installed in a sewing machine. At this time, the leader of the bobbin thread is manually wound on the shaft 25a of the bobbin 25 several times.
When the bobbin case is set in a rotary hook base, the bobbin 25 on which the bobbin thread Y has been wound is mounted in the bobbin case body 21. Then the winding end portion of the bobbin thread Y is led to the lower face of a tension spring 22 via the open groove 21a and pulled out via the thread guide portion 22a in the direction of an arrow (upward because those shown in FIGS. 3 and 7 use a vertical rotary hook portion. When the sewing operation is performed after the bobbin 25 is mounted in the rotary hook portion, the bobbin thread Y is pulled out as the sewing operation progresses and the bobbin 25 is rotated in the pulling direction as shown by the arrow in those figures.
Although the aforementioned bobbin case is designed to accommodate the bobbin 25 on which bobbin thread Y has been wound, there has been proposed an arrangement in which a "coreless cop" wound like a doughnut through a special winding method is directly accommodated without using a bobbin in order to prevent thread from coming apart (Japanese Utility Model Publication Nos. Sho. 44-17744 and No. 44-23020). A system of taking out thread in the axial direction like this is called a "longitudinal take-up system."
The examples of pulling out the bobbin thread Y while the bobbin 25 is being rotated as shown in FIGS. 3 and 7 cause the tension tempered with the weight of thread, the weight of the bobbin 25 and the clamping force of the tension spring 22 to be applied to the bobbin thread Y. However, the weight of thread varies between a case where the thread is fully wound on the bobbin 25 and a case where it almost runs out. Consequently, the bobbin thread tension when the thread has almost run out becomes halved from when the thread has fully been wound thereon. When, moreover, the winding diameter is reduced as the wound thread amount decreases at the time of sewing, an angle of rotation of the bobbin 25 gradually increases, though the bobbin thread take-up amount remains unchanged. Therefore, the coefficient of dynamic friction between the bobbin and the stud uprightly provided in the center of the rotary hook base is reduced, so that the bobbin thread tension is gradually decreased. When the bobbin thread Y is rapidly pulled out during the time the rotary hook portion is operated at a speed as high as about 8,000 r.p.m. (rotations per minute) or at the time of thread cutting, the bobbin 25 is allowed to rotate because of inertia force after the thread pulling operation is stopped. The slackening of thread is caused in the bobbin case because the bobbin thread Y has been excessively pulled out, which results in tension variation. Consequently, a kink, this is, twisting is added to the slackened thread and the wound thread may fall apart. In this case, the kink may cause the tension spring 22 to be tampered with thread, which results in an extreme tension variation. As variation in the bobbin thread tension is led to the lowering and rising of the connecting position with the upper thread, the variation in the bobbin thread tension as described above may originate in non-uniform seam. Particularly in the case of white shirt sewing, the allowed fluctuation width of bobbin thread tension is extremely small and the seam appearance is seriously affected. Although some bobbin case is equipped with a spring for preventing bobbin racing in its rotary hook base, the spring force has caused the bobbin thread tension to increase.
When the bobbin thread Y wound on the bobbin 25 decreases in amount as the sewing operation progresses, the leader of the thread interferes with the bobbin thread that is being released, which may cause fluctuation in the bobbin thread tension and may result in having the thread cut off sometimes in a locked condition.
On the other hand, what is adapted for directly accommodating thread in such a manner that the "coreless cop" is pulling out from the central part of the rotary hook portion makes the bobbin thread tension free from fluctuating as shown in FIGS. 3 and 7 but lets a kink (where thread becomes entwined around adjoining one) occur when the quantity of thread decreases. Although the user usually winds bobbin thread on a lower-thread bobbin using a lower-thread winding unit attached to a sewing machine, the "coreless cop" cannot be wound up in that way, whereupon the user will have to request a special manufacturer to do so. Consequently, such a "coreless cop" has not come into wide use yet because the cost therefor tends to mount up.